Method for uplink spectrum monitoring for sparse overlay tdma systems

ABSTRACT

Spectrum monitoring measurements are made by microcell base stations in a layered cellular network, while not serving calls or engaged in processing calls. The microcell base station transmits a first control message from to a microcell mobile station, to increase the duration for the mobile station to reside in the camping state on a control channel of the microcell base station. The microcell base station transmits a second control message from the microcell base station to the microcell mobile station, to increase the duration for the mobile station to reside in a call origination state while attempting to access a control channel of the microcell base station. Then, to perform the spectrum monitoring measurement, the base station&#39;s transmitter is turned off, the receiver is retuned to the frequency to be monitored, a signal strength measurement taken on that frequency, the receiver is retuned back to its assigned frequency, and the transmitter is turned back on, all in a short time interval. Because the measurement time is so short, it is possible to take spectrum-monitoring measurements without causing inactive mobiles registered on the measuring base station to reselect control channel. Mobiles initiating a call during a spectrum monitoring measurement by its base station are also delayed in their control channel reselection.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/913,546, filed Aug. 9, 2004 (currently allowed), which is acontinuation of Ser. No. 09/947,367 filed on Sep. 7, 2001 which hasissued as U.S. Pat. No. 6,792,268, where each of the above referencedapplications is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed broadly relates to wireless communications andmore particularly relates to RF spectrum monitoring for wirelesssystems.

2. Related Art

The invention disclosed is related to U.S. Pat. No. 6,615,040 toMathilde Benveniste, issued Sep. 2, 2003, entitled “Self-ConfigurableWireless Systems: Spectrum Monitoring In A Layered Configuration”,assigned to the AT&T Corporation, and incorporated herein by reference.

The invention disclosed is also related to U.S. Pat. No. 5,404,574 toMathilde Benveniste, issued Apr. 4, 1995, entitled “Apparatus And MethodFor Non-Regular Channel Assignment In Wireless Communication Systems”,assigned to the AT&T Corporation, and incorporated herein by reference.

The invention disclosed is also related to U.S. Pat. No. 5,809,423 toMathilde Benveniste, issued Sep. 15, 1998, entitled “Adaptive-DynamicChannel Assignment Organization System And Method”, assigned to LucentTechnologies, Inc., and incorporated herein by reference.

The invention disclosed is also related to U.S. Pat. No. 5,787,352 toMathilde Benveniste, issued Jul. 28, 1998, entitled “System and methodfor management of neighbor-channel interference with power control anddirected channel assignment”, assigned to the AT&T Corporation, andincorporated herein by reference.

3. Background

The IS-136 Digital Cellular/PCS Standard

An IS-136 Digital Cellular/PCS system includes one or more cellularradio telephones or mobile devices within the communications range of abase station. The base station can have one or more radio transceivers,a control computer, and an antenna system. The base station is connectedby means of a mobile switching center (MSC) to the public switchedtelephone network (PSTN). The mobile switching center provides thefunctionality of a telephone central office and is additionallyresponsible for call processing, mobility management, and radio resourcemanagement. The IS-136 Digital Cellular/PCS network architecture andsystem operation are specified by the Telecommunications IndustryAssociation, TIA/EIA Interim Standard: TDMA Cellular/PCS-RadioInterface-Mobile Station-Base Station Compatibility, Revision A,TIA/EIA/IS-136-A, October 1996.

FIG. 1 shows the prior art state diagram of an IS-136 compliant mobiledevice. A mobile's operation is carried out in processes that occur whenit transitions from one state to another. The processes are carried outby software in the mobile. State 102 is the null state when the mobileis powered down. Upon being powered up by the power up action path 120,the mobile transitions to state 104 which is the Control ChannelScanning and Locking state. A mobile executes the DCCH Scanning andLocking procedure while in this state, attempting to find a digitalcontrol channel (DCCH). A mobile executes the Control Channel Selectionprocedure from the Control Channel Scanning and Locking state 104 once aDCCH is found using the DCCH Scanning and Locking procedure. The ControlChannel Selection procedure is executed in order to determine if theDCCH is suitable for camping.

Communication between the mobile device and the base station employs atime division multiple access (TDMA) frame structure shown in FIG. 2,where each frame 200 and 200′ has six time slots. Frames 200 and 200′shown in FIG. 1, occur at different frame times in the same 30-kHz RFchannel. In the digital PCS full rate mode, each user is allowed accessto two of the six time slots in a frame, so one RF channel cansimultaneously serve up to three users. The frames have a duration of 40ms and are organized into superframes of 16 TDMA frames, having aduration of 640 ms. A hyperframe consists of two superframes, and has aduration of 1.28 seconds. Frequency division multiple access (FDMA) isused to support more users by assigning multiple RF channels for use inthe same cell. A mobile user transmits in the reverse or uplinkdirection on one frequency and receives in the forward or downlinkdirection on a different frequency, using frequency division duplexing(FDD).

Only one forward digital control channel (DCCH) consisting of two timeslots 202 and 208 in one RF channel of FIG. 2, is required in theforward or downlink direction from the base station to the mobiles.Similarly, only one reverse digital control channel (DCCH) consisting oftwo time slots in another RF channel, is required in the reverse oruplink direction from the mobiles to the base station. The logicalchannels of the DCCH in the forward direction (from the base station tothe mobile) are divided into broadcast channels and point-to-pointchannels, while the reverse DCCH is a point-to-point channel. The frame200 shown in FIG. 2 carries the forward digital control channel (DCCH)containing control channel selection parameters for SCANINTERVAL in timeslot 202 and DELAY VALUE in time slot 208. Time slots 204 and 210 carrytraffic from the base station (BS) to mobile 20 and time slots 206 and212 carry traffic from the base station (BS) to mobile 20′. The frame200A shown in FIG. 2 carries the forward digital control channel (DCCH)containing the access parameter for MAX_RETRIES in time slot 214. Timeslots 216 and 222 carry traffic from the base station (BS) to mobile 20and time slots 218 and 224 carry traffic from the base station (BS) tomobile 20′. Time slot 220 carries other forward digital control channel(DCCH) broadcast channel messages or point-to-point control channelmessages.

Once a suitable digital control channel (DCCH) is found, action path 121causes the mobile to transition to the DCCH Camping state 106 in FIG. 1.The DCCH Camping state 106 is the normal state for the mobile while inservice on a DCCH and not processing a call. Upon entering the DCCHCamping state, a mobile must read control information from the basestation contained in the DCCH messages 140. This information includescontrol channel selection parameters used by the mobile in thereselection of a DCCH, including the SCANINTERVAL parameter and theDELAY parameter. This information also includes access parameters usedby the mobile to access the base station, including the MAX_RETRIESparameter. The mobile periodically executes a number of processes andprocedures while in the DCCH Camping state 106, including controlchannel reselection. A mobile remains in the DCCH Camping state 106while it reselects from one DCCH to another. In the process reselectionto select the best DCCH, the mobile periodically measures the receivedsignal strength on the neighboring control channels from base stationsin neighboring cells. The mobile then evaluates whether another basestation's control channel would be better. If a better control channelis found, the mobile tunes to the new base station's control channel,synchronizes, and begins to monitor it for messages.

While in the DCCH Camping state 106, a mobile periodically measures thesignal strength on the current DCCH and all control channels in theneighbor cell list. This measurement process is called the ControlChannel Locking procedure, and is executed every SCANINTERVAL. TheControl Channel Locking procedure is part of the Control ChannelReselection procedure. The mobile must wait for a delay interval timedby its camping state delay timer 106A, whose maximum delay value is setby the DELAY parameter, before the mobile can begin periodicallymeasuring the signal strength. The DELAY parameter keeps the mobile fromconsidering a neighboring control channel as a reselection candidateuntil the time delay has been met.

Once the DELAY interval has expired in camping state delay timer 106A,the mobile executes the Control Channel Reselection procedure to find abetter control channel from which to obtain service. It typicallyinvokes this procedure from the DCCH Camping state 106 over action path132. In the Control Channel Locking procedure, the mobile measures andaverages the signal strength on the current DCCH and all neighboringcontrol channels. It does this periodically at the measurement intervaldetermined by the value of SCANINTERVAL broadcast on the current DCCH.The period is timed by the camping state scan timer 106B, whose maximumduration value is set by the SCANINTERVAL parameter. The signal strengthof the current DCCH is measured every SCANINTERVAL. The signal strengthof neighboring control channels is measured either every SCANINTERVAL orevery other SCANINTERVAL.

The mobile leaves the DCCH Camping state 106 in FIG. 1 to processtransactions such as registration in the Registration Proceeding state108, call origination in the Origination Proceeding state 112, orreceived call termination in the waiting for order state 110. Acondition that precipitates leaving the DCCH Camping state 106 andreturning to the Control Channel Scanning and Locking state 104 islosing service on the current DCCH, such as by a radio link failure onaction path 130 and being unable to find another control channel throughreselection. For further details, see the IS-136 Digital Cellular/PCSspecification, section 6.3.3.4.1. A mobile stays in the DCCH Campingstate 106 while it reselects from one DCCH to another.

The mobile transitions along action path 142 in FIG. 1 to theRegistration Proceeding state 108 after sending a Registration messageon a DCCH or in response to a forced registration request from the basestation. Registration is the process whereby a mobile identifies itselfto the base station and makes itself available for service. It remainsin the Registration Proceeding state 108 for up to sixteen seconds whilewaiting for a response from the base station to its registrationattempt. It returns from this state on path 142′ upon expiration of thesixteen second timer indicating that the time to wait has expired.

The mobile transitions along action path 122 in FIG. 1 to theOrigination Proceeding state 112 after sending an origination message ona DCCH and while waiting for a response from the base station. TheMAX_RETRIES parameter is the access parameter used by the mobile toattempt accessing the base station up to a maximum number of attempts.The MAX_RETRIES parameter sets the maximum retry count in the callorigination retry counter 112A. The mobile leaves the OriginationProceeding state 112 when it receives a digital traffic designation fromthe base station, and moves to the assigned channel along path 124. Ifthe mobile determines that the origination message was not received bythe base station, it returns to the DCCH Camping state 106.

The mobile transitions along action path 146 in FIG. 1 to the WaitingFor Order state 110 after sending a Page Response message on a DCCH andwhile waiting for a response from the base station. A page is a messagesent by the base station to inform the mobile that a message is waiting,either a voice message, a short message service (SMS) message, or a faxmessage. The mobile remains in Waiting For Order state 110 for up to 6.4seconds while waiting for a response from the base station. The mobileleaves the waiting for order state 110 when it receives a digitaltraffic designation from the base station, and moves to the assignedchannel along path 126. It returns from the waiting for order state 110along path 146′ upon expiration of the 6.4 second timer indicating thetime has expired to wait between sending a Page Response message andreceiving a response from the base station.

The mobile transitions along action path 144 in FIG. 1 to the SharedSecret Data (SSD) Update Proceeding state 114 if it has sent aConfirmation message in response to a Notification message indicating anSSD update. Shared Secret Data (SSD) is a 128-bit pattern stored in themobile's memory, used to support authentication, voice privacy, andmessage encryption. It leaves the SSD Update Proceeding state 114 uponexpiration of a twelve second timer indicating the time has expired towait between successfully sending a Confirmation message and receiving aresponse from the base station.

Overlay Networks

The IS-136 Digital Cellular/PCS system can form hierarchical cellstructures such as shown in FIG. 3, to increase the capacity of acellular system by positioning smaller area adjunct cells or microcells310 to operate within a system 300 of larger area primary cells ormacrocells 301, 302, and 303. Such cells exist in a layered or overlayconfiguration that enables greater re-use of the RF spectrum andprovides continuity of communication between the cell layers. An exampleis an indoor microcell system 310 operating within an outdoor macrocellsystem 300.

In an overlay configuration where a microcell wireless system 310 sharesthe same spectrum as the macrocell system 300 which is the owner of theRF spectrum, it is necessary for the microcell system 310 to monitor theactivity of the macrocell system 300 on all RF channels. This involvesperiodic signal strength measurements that are made both by mobiles 20and by the base stations 10 in the microcell wireless system 310. Themicrocell system 310 partitions all of the monitored RF channels intotwo sets: a first set of channels that are likely to beinterference-free and a second set of noisy channels. A list is compiledof the interference-free channels in the first set as a pool ofavailable channels that can be assigned to the microcell base stationsand their registered mobiles. In the above-referenced copending U.S.patent application Ser. No. 09/401,408, filed Sep. 22, 1999, to MathildeBenveniste, entitled “Self-Configurable Wireless Systems: SpectrumMonitoring In A Layered Configuration”, a mechanism is discussed toacquire such measurements on downlink channels by the mobiles 20, bydeploying the mobiles registered to the microcell system 310.

What is needed is a method for signal strength measurements on uplinkchannels by the microcell base stations 10. However, the measurement ofsignal strength on uplink channels by the microcell base stations 10must deal with a special set of problems when there are relatively fewmicrocell base stations 10 in a region of a registered mobile station20.

In a region where there is a high-density of microcell base stations 10available with overlapping coverage, mobiles 20 registered to an idlemicrocell base station 10 can be induced to register to a neighboringmicrocell base station while measurements are taken by the idle basestation 10. A powered-up mobile 20 registers when it enters the coveragearea of a base station 10, or when it is first powered on, and it isplaced on the list of registered mobiles. This enables the microcellbase station 10 to page it if there is an incoming call. An inactivemobile 20 [i.e., a mobile not engaged in a call] does not need a trafficchannel. It is assigned a traffic channel when a call starts.

When a microcell base station 10 is idle (that is, not serving anycalls), it can retune to different channels and take measurements ofseveral channels at once, thus offering an efficient mechanism forspectrum monitoring. This can be achieved by disabling the DigitalControl Channel (DCCH), thus forcing the registered mobiles 20 toreselect another control channel.

However, in sparse systems where there are relatively few microcell basestations 10 in a region, it is not desirable to cause a registeredmobile station 20 to undertake control channel reselection. Thecapability to stay on the current control channel is desirable for amicrocell system 310 because control channel reselection in a sparsesystem will likely force mobiles 20 to register on the outside,macrocell cellular system 300. Since return to the original controlchannel is not immediate, a mobile 20 could end up placing a call on theoutside, macrocell system 300 while the subscriber thinks he/she isbeing served by the microcell system 310.

Thus, the prior art is confronted with at least two problems. One is toavoid causing the inactive mobiles registered on the measuring microcellbase station to miss their control channel and thus be forced tore-select a macrocell base station. The second problem is to reduce theprobability that a call initiated by a mobile during an uplinkspectrum-monitoring measurement by a microcell base station, will causethe transfer of the call to a macrocell base station.

SUMMARY OF THE INVENTION

A method and system are disclosed to avoid causing the inactive mobilesregistered on a measuring microcell base station to miss their controlchannel and thus be forced to re-select a macrocell base station. Theinvention reduces the probability that a call initiated by a mobileduring an uplink spectrum-monitoring measurement by a microcell basestation, will cause the transfer of the call to a macrocell basestation.

The method of the invention begins with the microcell base stationdetermining that there is no traffic with mobile stations in themicrocell. Then the microcell base station gets the estimated durationfor the next spectrum monitoring measurement session. The actualduration required to complete a spectrum monitoring measurement sessionis a function of the number channels in active use in both the macrocellsystem and in the microcell system. This is a function of the diurnaland weekly calling habits and movement patterns of the users. This isalso a function of diurnal and weekly solar activity, diurnalinterference from out-of-band users, and other environmental factorsthat vary with time. The aggregate effect of these factors on theavailability of interference-free channels manifests itself in thespectrum monitoring measurement. The length of time to carry out thespectrum monitoring measurement is recorded during each measurementsession, and this updated value is used as the estimated duration of thenext spectrum monitoring measurement session.

Then the microcell base station transmits a digital control channel(DCCH) message to the mobile stations to set the “DELAY” parameter fortheir camping state delay timers to be greater than the estimatedduration value. The mobile must wait for a delay interval set by theDELAY parameter before it can begin periodically measuring the signalstrength of other digital control channels. The DELAY parameter keepsthe mobile from considering a neighboring digital control channel as areselection candidate until the time delay has been met. Since the delayis forced by the base station to be longer than the estimated durationof the spectrum monitoring measurement session, the mobile station iseffectively prevented from leaving the base station's digital controlchannel (DCCH). As an alternative, the “DELAY” parameter can be set toits maximum value of 4.5 minutes.

The ability of the base station to prevent the mobile from leaving theDCCH is enhanced by forcibly resetting the mobile station's campingstate delay timer to zero. There are three alternate ways to accomplishthis.

Then the microcell base station can transmit a page notification in aDCCH message, without a follow up. This will force the mobile out of thecamping state for 6.4 secs and reset mobile's camping state delay timerto zero. As an alternate way, the microcell base station can transmit anSSD update notification in a DCCH message. This will force the mobileout of the camping state for 12 secs and reset mobile's camping statedelay timer to zero. As a further alternate way, the microcell basestation can transmit a forced re-registration request in a DCCH message.This will force the mobile out of the camping state for 16 secs andreset mobile's camping state delay timer to zero.

Then the microcell base station turns off the base station'stransmitter. It then retunes the base station's receiver to thefrequency to be monitored. It then measures a signal strength on thatfrequency. It then retunes the receiver back to its assigned frequency.The microcell base station then turns on the transmitter withinestimated duration.

Then the microcell base station transmits a DCCH message to the mobilestations to reset the “DELAY” parameter for their camping state delaytimers to be normal duration value.

Finally, the microcell base station updates the estimated duration forspectrum monitoring measurements. The microcell base station thenreturns to the first step to wait until another interval occurs whenthere is no traffic with mobile stations in the microcell.

In an alternate embodiment of the invention, the microcell base stationtransmits a digital control channel (DCCH) message to the mobilestations to set the “SCANINTERVAL” parameter for their camping statescan timers to be greater than the estimated duration value. Once theDELAY interval has expired, the mobile begins the periodic scanning ofother DCCHs to find a better control channel from which to obtainservice. The mobile periodically measures and averages the signalstrength on the current DCCH and all neighboring control channels. Itdoes this after the periodic measurement interval determined by thevalue of SCANINTERVAL. Since the periodic scanning is forced by the basestation to begin after SCANINTERVAL, which is longer than the estimatedduration of the spectrum monitoring measurement session, the mobilestation is effectively prevented from leaving the base station's digitalcontrol channel (DCCH). When the spectrum monitoring measurements arecompleted, the microcell base station transmits a DCCH message to themobile stations to reset the “SCANINTERVAL” parameter for their campingstate scan timers to be normal duration value. As an alternative, the“SCANINTERVAL” parameter can be set to its maximum value of 20.5seconds.

In a further alternate embodiment of the invention, the microcell basestation transmits a digital control channel (DCCH) message to the mobilestations to set the “MAX_RETRIES” parameter for their call originationretry counters to count for longer than the estimated duration. TheMAX_RETRIES parameter is the access parameter used by the mobile toattempt accessing the base station up to a maximum number of attempts.If the user of the mobile were to originate a call during the spectrummonitoring measurement session, the mobile will need the use of the DCCHto set up the call with the base station. By forcing the MAX_RETRIESparameter to be large, the mobile will repeatedly go through the processof retrying to access the DCCH so as to take longer than the estimatedduration for the spectrum monitoring measurement session. When thespectrum monitoring measurements are completed, the microcell basestation transmits a DCCH message to the mobile stations to reset the“MAX_RETRIES” parameter for their call origination retry counters to benormal count value. As an alternative, the “MAX_RETRIES” parameter canbe set to its maximum value of 7 retries.

The resulting invention avoids causing inactive mobiles registered on ameasuring microcell base station to miss their control channel and thusbe forced to re-select a macrocell base station.

DESCRIPTION OF THE FIGURES

FIG. 1 is a prior art state diagram for the operation of a wirelessmobile device which is compliant with the IS-136 standard.

FIG. 2 is a prior art diagram of IS-136 frames carrying a forwarddigital control channel (DCCH) containing control channel selection andaccess parameters.

FIG. 3 shows a system diagram of an indoor microcell wireless systemwithin a an outdoor macrocell system, in accordance with the invention.

FIG. 4 illustrates the state diagram for the operation of a wirelessmobile device in accordance with the invention.

FIG. 5A is a diagram of IS-136 frames carrying forward digital controlchannel (DCCH) containing messages to set timer and counter values tomaximum in the mobile device, in accordance with the invention.

FIG. 5B is a diagram of IS-136 frames carrying forward digital controlchannel (DCCH) containing messages to reset Camping state delay timer tozero in the mobile device, in accordance with the invention.

FIG. 6A is a flow diagram of the sequence of operational steps for amicrocell base station transmitting a large value for the “DELAY”parameter to assist in making uplink spectrum monitoring measurements,in accordance with the invention.

FIG. 6B is a flow diagram of the sequence of operational steps for amicrocell base station transmitting a large value for the “SCANINTERVAL”parameter to assist in making uplink spectrum monitoring measurements,in accordance with the invention.

FIG. 6C is a flow diagram of the sequence of operational steps for amicrocell base station transmitting a large value for the “MAX_RETRIES”parameter to assist in making uplink spectrum monitoring measurements,in accordance with the invention.

FIG. 6D is a flow diagram of the sequence of operational steps for amicrocell base station transmitting large values for both the “DELAY”parameter and the “MAX_RETRIES” parameter to assist in making uplinkspectrum monitoring measurements, in accordance with the invention.

FIG. 6E is a flow diagram of the sequence of operational steps for amicrocell base station transmitting large values for both the“SCANINTERVAL” parameter and the “MAX_RETRIES” parameter to assist inmaking uplink spectrum monitoring measurements, in accordance with theinvention.

DISCUSSION OF THE PREFERRED EMBODIMENT

The operation of a wireless mobile device 20 which is registered to amicrocell base station 10 is controlled by the base station as shown inthe state diagram of FIG. 4, in accordance with the invention. Thevarious states depicted in FIG. 4 are the same as those described forFIG. 1, but the particular control inputs to the states and theapplication of the responses by the states are modified, in accordancewith the invention. The microcell base station 10, transmits digitalcontrol channel (DCCH) inputs 140′ to the mobile 20 which prevent themobile 20 from reselecting a control channel while microcell basestation 10 does uplink monitoring.

FIG. 5A is a diagram of IS-136 frames carrying forward digital controlchannel (DCCH) containing messages to set timer and counter values tolarge values in the mobile device, in accordance with the invention. Theframe 200A shown in FIG. 5A contains control channel selectionparameters for a larger SCANINTERVAL value in time slot 202A and alarger DELAY VALUE in time slot 208A. Time slots 204 and 210 carrytraffic from the base station (BS) to a first mobile 20 and time slots206 and 212 carry traffic from the base station (BS) to a second mobile20′. The frame 200A shown in FIG. 2 carries the forward digital controlchannel (DCCH) containing a larger value for the access parameterMAX_RETRIES in time slot 214A. Time slots 216 and 222 carry traffic fromthe base station (BS) to mobile 20 and time slots 218 and 224 carrytraffic from the base station (BS) to mobile 20′. Time slot 220 carriesother forward digital control channel (DCCH) broadcast channel messagesor point-to-point control channel messages.

The microcell base station 10 transmits a digital control channel (DCCH)message 208A in FIG. 5A to the mobile stations 20 to set the “DELAY”parameter for their camping state delay timers 106A to be greater thanthe estimated duration value. The mobile must wait for a delay intervalset by the DELAY parameter before it can begin action path 132 of FIG.4, to periodically measure the signal strength of other digital controlchannels. The DELAY parameter keeps the mobile 20 from considering aneighboring digital control channel as a reselection candidate until thetime delay has been met. Since the delay is forced by the base station10 to be longer than the estimated duration of the spectrum monitoringmeasurement session, the mobile station 20 is effectively prevented fromleaving the base station's digital control channel (DCCH). When thespectrum monitoring measurements are completed, the microcell basestation transmits a DCCH message to the mobile stations to reset the“DELAY” parameter for their camping state delay timers 106A to be normalduration value. As an alternative, the “DELAY” parameter can be set toits maximum value of 4.5 minutes.

The microcell base station 10 transmits a digital control channel (DCCH)message 202A in FIG. 5A to the mobile stations 20 to set the“SCANINTERVAL” parameter for their camping state scan timers 106B to begreater than the estimated duration value. Once the DELAY interval hasexpired, the mobile 20 can start the scan timer 106B to periodicallyscan other DCCHs to find a better control channel from which to obtainservice. The mobile 20 periodically measures and averages the signalstrength on the current DCCH and all neighboring control channels. Itbegins action path 132 of FIG. 4, after the periodic measurementinterval determined by the value of SCANINTERVAL. Since the periodicscanning is forced by the base station 10 to begin after SCANINTERVAL,which is longer than the estimated duration of the spectrum monitoringmeasurement session, the mobile station 20 is effectively prevented fromleaving the base station's digital control channel (DCCH). When thespectrum monitoring measurements are completed, the microcell basestation transmits a DCCH message to the mobile stations to reset the“SCANINTERVAL” parameter for their camping state scan timers 106B to benormal duration value. As an alternative, the “SCANINTERVAL” parametercan be set to its maximum value of 20.5 seconds.

The microcell base station 10 transmits a digital control channel (DCCH)message 214A in FIG. 5A to the mobile stations 20 to set the“MAX_RETRIES” parameter for their call origination retry counters 112Ato count for longer than the estimated duration. The MAX_RETRIESparameter is the access parameter used by the mobile 20 to attemptaccessing the base station 10 up to a maximum number of attempts. If theuser of the mobile 20 were to originate a call during the spectrummonitoring measurement session, the mobile 20 will need the use of theDCCH to set up the call with the base station 10. By forcing theMAX_RETRIES parameter to be large, the mobile 20 will repeatedly gothrough the process of retrying to access the DCCH so as to take longerthan the estimated duration for the spectrum monitoring measurementsession. When the spectrum monitoring measurements are completed, themicrocell base station 10 transmits a DCCH message to the mobilestations 20 to reset the “MAX_RETRIES” parameter for their callorigination retry counters 112A to be normal count value. As analternative, the “MAX_RETRIES” parameter can be set to its maximum valueof 7 retries.

The ability of the base station 10 to prevent the mobile 20 from leavingthe DCCH is enhanced by forcibly resetting the mobile station's campingstate delay timer 106A to zero. There are three alternate ways toaccomplish this, as shown in FIG. 4. The IS-136 frame in FIG. 5B,carries a forward digital control channel (DCCH) containing messages toreset Camping state delay timer to zero in the mobile device, inaccordance with the invention. The microcell base station 10 cantransmit a page notification 146B in a DCCH message 240 of FIG. 5B,without a follow up. This will force the mobile 20 out of the campingstate 106 for 6.4 secs as shown in FIG. 4, and will reset mobile'scamping state delay timer 106A to zero.

As an alternate way, the microcell base station 10 can transmit an SSDupdate notification 144B in a DCCH message 240 of FIG. 5B. This willforce the mobile 20 out of the camping state 106 for 12 secs as shown inFIG. 4, and will reset mobile's camping state delay timer 106A to zero.

As a further alternate way, the microcell base station 10 can transmit aforced re-registration request 142B in a DCCH message 240 of FIG. 5B.This will force the mobile 20 out of the camping state 106 for 16 secsas shown in FIG. 4, and will reset mobile's camping state delay timer106A to zero.

The IS-136 frame in FIG. 5B, has other time slots 236 and 242 that carrytraffic from the base station (BS) to mobile 20 and time slots 238 and244 that carry traffic from the base station (BS) to mobile 20′. Timeslot 234 carries other forward digital control channel (DCCH) broadcastchannel messages or point-to-point control channel messages.

FIG. 6A is a flow diagram of the method 600A which is a sequence ofoperational steps for a microcell base station transmitting a largevalue for the “DELAY” parameter to assist in making uplink spectrummonitoring measurements, in accordance with the invention. The firststep 602 is for the microcell base station to determine that there is notraffic with mobile stations in the microcell.

Then in step 604, the microcell base station gets the estimated durationfor the next spectrum monitoring measurement session. The actualduration required to complete a spectrum monitoring measurement sessionis a function of the number channels in active use in both the macrocellsystem and in the microcell system. This is a function of the diurnaland weekly calling habits and movement patterns of the users. This isalso a function of diurnal and weekly solar activity, diurnalinterference from out-of-band users, and other environmental factorsthat vary with time. The aggregate effect of these factors on theavailability of interference-free channels results in the spectrummonitoring measurement. The length of time to carry out the spectrummonitoring measurement is recorded during each measurement session, andis used as the estimated duration of the next spectrum monitoringmeasurement session.

Then in step 606A, the microcell base station transmits a digitalcontrol channel (DCCH) message 208 a to the mobile stations to set the“DELAY” parameter for their camping state delay timers to be greaterthan the estimated duration value. The mobile must wait for a delayinterval set by the DELAY parameter before it can begin periodicallymeasuring the signal strength of other digital control channels. TheDELAY parameter keeps the mobile from considering a neighboring digitalcontrol channel as a reselection candidate until the time delay has beenmet. Since the delay is forced by the base station to be longer than theestimated duration of the spectrum monitoring measurement session, themobile station is effectively prevented from leaving the base station'sdigital control channel (DCCH). As an alternative, the “DELAY” parametercan be set to its maximum value of 4.5 minutes.

The ability of the base station to prevent the mobile from leaving theDCCH is enhanced by forcibly resetting the mobile station's campingstate delay timer to zero. There are three alternate ways to accomplishthis, each having a different time away from the camping state 106, thethree ways being selected by step 608.

In step 610, the microcell base station can transmit a page notificationin a DCCH message 146 b, without a follow up. This will force the mobileout of the camping state for 6.4 secs and reset mobile's camping statedelay timer to zero.

In step 612, as an alternate way, the microcell base station cantransmit an SSD update notification in a DCCH message 144 b. This willforce the mobile out of the camping state for 12 secs and reset mobile'scamping state delay timer to zero.

In step 614, as a further alternate way, the microcell base station cantransmit a forced re-registration request in a DCCH message 142 b. Thiswill force the mobile out of the camping state for 16 secs and resetmobile's camping state delay timer to zero.

In step 616, the microcell base station turns off the base station'stransmitter.

In step 618, the microcell base station then retunes the base station'sreceiver to the frequency to be monitored.

In step 620, the microcell base station then measures a signal strengthon that frequency.

In step 622, the microcell base station then retunes the receiver backto its assigned frequency.

In step 624, the microcell base station then turns on the transmitterwithin estimated duration.

In step 626A, the microcell base station transmits a DCCH message to themobile stations to reset the “DELAY” parameter for their camping statedelay timers to be normal duration value.

Finally, In step 628, the microcell base station updates the estimatedduration for spectrum monitoring measurements. The microcell basestation then returns to the first step 602 to wait until anotherinterval occurs when there is no traffic with mobile stations in themicrocell.

In an alternate embodiment of the invention shown in the method 600B ofFIG. 6B, the microcell base station transmits a digital control channel(DCCH) message 202A in step 606B to the mobile stations to set the“SCANINTERVAL” parameter for their camping state scan timers to begreater than the estimated duration value. Once the DELAY interval hasexpired, the mobile begins the periodic scanning of other DCCHs to finda better control channel from which to obtain service. The mobileperiodically measures and averages the signal strength on the currentDCCH and all neighboring control channels. It does this after theperiodic measurement interval determined by the value of SCANINTERVAL.Since the periodic scanning is forced by the base station to begin afterSCANINTERVAL, which is longer than the estimated duration of thespectrum monitoring measurement session, the mobile station iseffectively prevented from leaving the base station's digital controlchannel (DCCH). When the spectrum monitoring measurements are completed,the microcell base station transmits a DCCH message to the mobilestations in step 626B to reset the “SCANINTERVAL” parameter for theircamping state scan timers to be normal duration value. As analternative, the “SCANINTERVAL” parameter can be set to its maximumvalue of 20.5 seconds. In FIG. 6B, the steps 602, 604, 608 to 624, and628 are the same as those described for FIG. 6A.

In a further alternate embodiment of the invention shown in the method600C of FIG. 6C, the microcell base station transmits a digital controlchannel (DCCH) message 214A in step 606C to the mobile stations to setthe “MAX_RETRIES” parameter for their call origination retry counters tocount for longer than the estimated duration. The MAX_RETRIES parameteris the access parameter used by the mobile to attempt accessing the basestation up to a maximum number of attempts. If the user of the mobilewere to originate a call during the spectrum monitoring measurementsession, the mobile will need the use of the DCCH to set up the callwith the base station. By forcing the MAX_RETRIES parameter to be large,the mobile will repeatedly go through the process of retrying to accessthe DCCH so as to take longer than the estimated duration for thespectrum monitoring measurement session. When the spectrum monitoringmeasurements are completed, the microcell base station transmits a DCCHmessage in step 626C to the mobile stations to reset the “MAX_RETRIES”parameter for their call origination retry counters to be normalduration value. As an alternative, the “MAX_RETRIES” parameter can beset to its maximum value of 7 retries. In FIG. 6C, the steps 602, 604,608 to 624, and 628 are the same as those described for FIG. 6A.

FIG. 6D is a flow diagram of the sequence of operational steps for amicrocell base station transmitting large values for both the “DELAY”parameter in step 606A and the “MAX_RETRIES” parameter in step 606C toassist in making uplink spectrum monitoring measurements, in accordancewith the invention. Steps 626A and 626C reset these parameters to theirnormal values when the spectrum monitoring measurements are completed.In FIG. 6D, the steps 602, 604, 608 to 624, and 628 are the same asthose described for FIG. 6A.

FIG. 6E is a flow diagram of the sequence of operational steps for amicrocell base station transmitting large values for both the“SCANINTERVAL” parameter in step 606B and the “MAX_RETRIES” parameter instep 606C to assist in making uplink spectrum monitoring measurements,in accordance with the invention. Steps 626B and 626C reset theseparameters to their normal values when the spectrum monitoringmeasurements are completed. In FIG. 6E, the steps 602, 604, 608 to 624,and 628 are the same as those described for FIG. 6A.

Detailed Discussion of the Invention

1. Control Channel Re-Selection During Spectrum Monitoring

A Spectrum Monitoring measurement on an arbitrary frequency involves thefollowing steps, as shown in FIG. 6A:

-   (1) Step 616: the transmitter is turned off;-   (2) Step 618: the receiver/transmitter is tuned to the frequency to    be measured, and a timer is set to allow sufficient time for the    retuning operation to complete;-   (3) Step 620: once the timer expires, the measurement is made;-   (4) Step 622: once the measurement is completed, the    receiver/transmitter is tuned back to the original control frequency    and a timer is set to allow sufficient time for the retuning    operation to complete; and-   (5) Step 624: once the timer expires, the transmitter is turned back    on.    The above steps are collectively referred to as the “measurement    procedure”.

If the duration of the above measurement procedure is sufficiently long,control channel re-selection in the base station system could betriggered by two conditions: Radio Link Failure and Periodic Scanning.(For further details, see the IS-136 Digital Cellular/PCS specification,section 6.3.3.4.1.) The first condition is of less concern than thesecond, because it takes longer to trigger re-selection. The firstcondition takes 10 hyper-frames (12.8 secs) to trigger. (For furtherdetails, see the IS-136 Digital Cellular/PCS specification, section5.5.1.) The second condition triggers re-selection after a time intervalthat is less than or equal to the parameter SCANINTERVAL, which is themeasurement interval between periodic signal-strength measurements.Provided that the measurement procedure takes less than 12.8 secs, thefirst condition is never met. But there is always some likelihood thatthe second condition will be met and, thus, cause a registered mobile toselect a new control channel.

The probability of control-channel re-selection is related to theduration of the measurement procedure and the time intervalSCANINTERVAL. A registered mobile will re-select its control channelwhen its control channel vanishes, provided it has camped on its currentcontrol channel for a time interval equal to the value of the parameterDELAY, which may be set at a maximum value of 420 super-frames (4.5min). (For further details, see the IS-136 Digital Cellular/PCSspecification, section 6.5.) Hence, once the mobile has camped on acontrol channel for more than 4.5 min, the probability ofcontrol-channel reselection is equal toPr[resel]=measurement_length/SCANINTERVAL   Equation (1)

To reduce the likelihood that the second condition will be met, theparameter SCANINTERVAL can be set to the maximum value allowed, which is16 hyper-frames (20.5 secs). (For further details, see the IS-136Digital Cellular/PCS specification, section 6.5.)

It has been estimated that the measurement, including retuning, that is,steps 618 through the start of step 624, takes less than 600 ms. Thisestimate has been confirmed empirically by recording the base stationclock time at every step of the measurement procedure. Thus, when themeasurement length is 600 ms, the re-selection probability will be 3percent.

2. Time Off the Control Channel

A mobile that has re-selected control channel during a spectrummonitoring measurement will not return to its previous control channelimmediately after the transmitter of the measuring base station isturned back on. There will be a delay that will depend on the parametersin the system of the new control channel.

Experiments have been conducted that caused a mobile registered to abase station to re-select a control channel during a spectrum monitoringmeasurement, and found that the control channel re-selected belongs tothe macrocell system outside. It is expected that the outside macrocellsystem will be selected when the base station system is sparse. The timewas recorded when the mobile monitor displays a new control channel, andfound that it takes approximately 15 secs for a mobile that hasre-selected control channel to return to its original control channel,even if the original control channel is available sooner.

3. Preventing Control Channel Re-Selection

For systems employing the IS 136 Standard, the possibility of channelre-selection during uplink spectrum monitoring is eliminated altogetherby having the microcell base station 10 reset the mobile station'scamping state delay timer 106A (DELAY_TMR) for starting periodicscanning in the Camping state 106. A registered mobile 20 will re-selecta control channel when its current control channel vanishes, provided ithas camped on its current control channel for a time interval equal tothe value of the parameter DELAY. Setting the value of DELAY equal to420 super-frames (4.5 min) suspends the periodic scanning for a periodof 4.5 minutes.

The mobile station's camping state delay timer 106A (DELAY_TMR) is resetby forcing the mobile 20 to leave the Camping state 106 temporarily.There are several ways to so. Among them are:

-   1. Send a page notification to a mobile without follow up. (For    further details, see the IS-136 Digital Cellular/PCS specification,    section 6.2.6).-   2. Send an SSD update notification to the mobile. (For further    details, see the IS-136 Digital Cellular/PCS specification, section    6.2.8.)-   3. Send a forced re-registration request to the mobile in an audit    order. (For further details, see the IS-136 Digital Cellular/PCS    specification, section 6.3.7.)

When a page notification is sent to the mobile, for instance, the mobilewill enter the Waiting-for-Order state 110 and, after waiting for aninterval of 6.4 seconds it will return to the Camping state. The mobilestation's camping state delay timer 106A (DELAY_TMR) will bere-initialized at this point. If the DELAY parameter is set to a valuegreater than the spectrum-monitoring measurement duration, the periodicscanning will not occur during the measurement. This will allow thesystem to make spectrum monitoring measurements without everre-selecting the control channel.

The advantage of the page notification approach over the SSD update andthe forced re-registration approaches, is that the page notificationapproach takes 6.4 secs, as compared to 12 seconds needed for the SSDupdate approach and 16 secs needed for re-registration approach. It isimportant to keep the duration of all operations as short as possiblebecause of the impact on the sample spacing, i.e., the time betweenconsecutive measurements on the same channel. Maintaining appropriatesample spacing maximizes the system's ability to quickly and accuratelyrespond to interference changes.

4. Call Transfer Probability During Spectrum Monitoring

While a microcell base station 10 is monitoring the RF channels andupdating its list of available interference-free channels, its controlchannel is inactive for a short time interval. To find its assignedcontrol channel, a mobile station 20 initiating a call is allowed aquantity of Max Retries+1 attempts. (For further details, see the IS-136Digital Cellular/PCS specification, section 5.3.3.1.) If unsuccessful,the mobile will re-select a control channel, thus causing the call to betransferred to, and routed through, the nearest alternate base station.

The probability of call transfer is related to the number of accessretries allowed. Hence, in order to minimize the call transferprobability, the Max Retries is set equal to the maximum value allowed,which is equal to 7. The call transfer probability will depend on themeasurement duration as it affects both the probability that a call willbe started during the measurement, and the probability that the numberof access retries allowed will expire before the DCCH becomes availableagain. It has been calculated that, for an offered load of 0.381Erlangs, for instance, the call transfer probability is less than0.0000058 if the measurement duration is 0.6 sec. That is, on theaverage, one out of at least 172,000 measurements would cause a call tobe transferred. It is concluded therefore that the probability offorcing a call transfer to another base station during an uplinkspectrum monitoring measurement is negligible, when using the invention.

Various illustrative examples of the invention have been described indetail. In addition, however, many modifications and changes can be madeto these examples without departing from the nature and spirit of theinvention.

1. A method for performing measurements, comprising: transmitting acontrol message from a microcell base station to a microcell mobilestation, for increasing a duration for the mobile station to reside in acamping state on a control channel of the microcell mobile station;turning off a transmitter of the microcell base station; retuning areceiver of the microcell base station to a frequency to be monitored;measuring a signal strength on that frequency; retuning the receiverback to an assigned frequency, and turning on the transmitter.
 2. Themethod of claim 1, whereby measurements from said measuring are madewithout causing inactive mobile stations registered on the microcellbase station to reselect another control channel.
 3. The method of claim1, wherein said control message set a camping state delay timer to havea delay duration not less than an estimated measurement duration.
 4. Themethod of claim 3, wherein said transmitting transmits a second controlmessage from the microcell base station to the microcell mobile station,to reset said camping state delay timer, to begin said delay durationbefore reselecting another control channel, where said delay durationcausing periodic scanning of another control channel to be suspended forat least a desired measurement duration by forcing the mobile stationtemporarily out of the camping state.
 5. The method of claim 4, whereinsaid second control message is a page notification to the mobilestation, without follow up, where said page notification forcing themobile station out of the camping state temporarily.
 6. The method ofclaim 4, wherein said second control message is an SSD updatenotification to the mobile station, where the SSD update notificationforcing the mobile station out of the camping state temporarily.
 7. Themethod of claim 4, wherein said second control message is a forcedre-registration request to the mobile station, where said forcedre-registration request forcing the mobile station out of the campingstate temporarily.
 8. The method of claim 3, further comprising:updating said estimated measurement duration; transmitting a subsequentcontrol message, to set the delay duration for the mobile station toreside in the camping state to be not less than said updated estimatedmeasurement duration; and performing a subsequent measurement.
 9. Themethod of claim 1, wherein said control message sets a camping statescan timer to have a scan period duration not less than an estimatedmeasurement duration.
 10. The method of claim 9, wherein the scan periodduration has a value set to a maximum value in order to minimize aprobability that a new call by the mobile station will be set up on acell in the macrocell system following reselection of another controlchannel.
 11. The method of claim 10, wherein said transmitting transmitsa second control message from the microcell base station to themicrocell mobile station, to reset said camping state scan timer tobegin said scan period before reselecting another control channel, wheresaid begin said scan period causing periodic scanning of another controlchannel to be suspended for at least the value of scan period durationby forcing the mobile station temporarily out of the camping state. 12.A method for performing measurements, comprising: transmitting a controlmessage from a microcell base station to a microcell mobile station, forincreasing a duration for the mobile station to reside in a callorigination state while attempting to access a control channel of themicrocell base station; turning off a transmitter of the microcell basestation; retuning a receiver of the microcell base station to afrequency to be monitored; measuring a signal strength on thatfrequency; retuning the receiver back to an assigned frequency; andturning on the transmitter.
 13. The method of claim 12, wherebymeasurements of said measuring are made without causing inactive mobilestations registered on the microcell base station to reselect anothercontrol channel.
 14. The method of claim 12, wherein said controlmessage sets a Max Retries maximum number of attempts to access acontrol channel to be a maximum value in order to minimize a probabilitythat a new call by the mobile station will be set up on a cell in themacrocell system following reselection of another control channel.
 15. Amethod for performing spectrum monitoring, comprising: determining anabsence of traffic with a plurality of mobile stations in a microcell;measuring and providing a duration value of an estimated spectrummonitoring measurement; transmitting a message on a digital controlchannel (DCCH) to the plurality of mobile stations to set a delayparameter for a mobile station camping state delay timer to be greaterthan the duration value; and selecting a mobile station time away from acamping state to enable spectrum monitoring measurements to be made bythe microcell base station without causing inactive mobile stationsregistered on the microcell base station to reselect another controlchannel.
 16. The method of claim 15, further comprising: retuning amicrocell base station receiver to a frequency to be monitored andmeasured with return to the frequency, after the measurement; anddirecting the mobile stations to reset the delay parameter for theircamping state delay timers to a normal duration value wherebyspectrum-monitoring measurements are made without causing inactivemobile stations registered on the microcell base station to reselectanother control channel.
 17. The method of claim 15, wherein a pagenotification is included in the message without a follow up to force themobile station out of the camping state and reset a mobile stationcamping state delay timer to zero.
 18. The method of claim 15, wherein aSSD update notification is included in the message to force the mobilestation out of the camping state and reset the mobile station campingstate delay timer to zero.
 19. The method of claim 15, wherein a forcedre-registration request is included in the message to force the mobilestation out of the camping state and reset the mobile station campingdelay timers to zero.
 20. The method of claim 15, further comprising:updating said estimated measurement duration; transmitting a subsequentcontrol message, to set the delay duration for the mobile station toreside in the camping state to be not less than said updated estimatedmeasurement duration; and performing a subsequent spectrum monitoringmeasurement.
 21. A method, comprising: receiving at a mobile station afirst control message from a microcell base station to increase aduration value for the mobile station to reside in a camping state on acontrol channel of the microcell base station; starting a delay timer inthe mobile station for the increased duration value; and receiving asecond control message at the mobile station to reset the delay timer toa value whereby spectrum-monitoring measurements are made withoutimpacting a service provided by a microcell wireless system.